What does the 0-100 slider for Softness represent? I see Spacing and Fadeout show a % figure, but Softness (and Transparency) does not have the % attached, so I assume the numbers are not a strict percentage of the radius of the tool which gradates outwards to the radius of the tool (in other words at 50, the inner solid area is 50% of the radius, then it gradually goes light to the full radius size).

As you guessed, softness determines how far out the brush is solid. Then the falloff from there to the radius follows a cosine function. The omission of the % sign was an oversight and has no special significance.